8.1A: Oxidation and Reduction

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The simplest possible chemical reaction involves the transfer of one or more electrons between two atoms. In the simple reaction:

\[\ce{2 Na + Cl2 -> 2 NaCl}\]

we assume that sodium transfers an electron to chlorine creating the sodium ion and the chloride ion. We can write the two steps of this reaction as:

\[\ce{Na -> Na+1 + e-1}\]

and

\[\ce{Cl2 + 2 e-1 > 2 Cl-1 }\]

The first of these steps involves the loss of an electron and is called an oxidation reaction, while the second step involves the gain of an electron and is called a reduction reaction. The simple nemonic "Leo goes Ger" or Loss of electrons - oxidation; Gain of electrons - reduction has been used by several generations of students to remember these definitions. (OK! This is Simba, my apologies to Lion King aficionados.) Oxidation and reduction reactions (usually called redox reactions) are important in biochemistry and important families of enzymes are called reductases or oxidases based on their reduction or oxidation catalysis. We recognize ethanol as common drinking alcohol, but most people don't realize that ethanol is generated by fermentation reactions that naturally take place in the intestines. The body always has a small amount of ethanol being transported across the intestine walls. As a result, the body has an enzyme to metabolize ethanol called ethanol oxidase. (You might want to think about this. The body didn't invent ethanol to compensate for the consumption of fermented beverages like beer and wine since there are relatively recent on an evolutionary time scale. Ethanol oxidase is present in the body because it had to be there to compensate for the background ethanol. I would suspect that all mammals have this enzyme, not just people. Other things that are ingested by people for their psychotropic effects do not necessarily have enzymes for their detoxification, thus they stay in the tissues longer and have a greater potential for doing damage. If you follow my dictum of not consuming anything for which you don't have an enzyme, you'll be relatively safe.) Redox chemistry also forms the basis of corrosion chemistry and battery chemistry (electrochemistry). Simple electrochemical reactions If we take a piece of zinc metal and put it into a beaker of HCl, we will quickly notice the formation of bubbles on the surface of the zinc. Were we to leave the zinc in the acid for several minutes, we'd easily observe that the zinc is being dissolved while a gas is being liberated. This process is illustrated below: The reaction taking place here is obviously:

\[Zn + 2 H+1 ------> Zn+2 + H2\]

In this reaction, the hydrogen ions are being reduced while the zinc is being oxidized. A similar reaction may be observed if we place a zinc bar into a solution containing copper sulfate. Here the reaction is:

\[Zn + Cu+2 -----> Cu + Zn+2\]

Again the zinc is being oxidized while the copper is being reduced. Unlike the case of zinc in acid, the reaction will only continue until the copper has formed a film on the surface of the zinc, at which point the reaction stops since zinc ions are no longer able to escape to the solution.

Rules for assigning oxidation numbers:

The oxidation number of a free element = 0.
The oxidation number of a monatomic ion = charge on the ion.
The oxidation number of hydrogen = + 1 and rarely - 1.
The oxidation number of oxygen = - 2 and in peroxides - 1.
The sum of the oxidation numbers in a polyatomic ion = charge on the ion. Elements in group 1, 2, and aluminum are always as indicated on the periodic table.